Wrong totally a bad idea if you are wanting to put a switch to go across the 12v rails to stop the 12v current running into the 12-5v converter then you will need to solder the wires to positive( Ie...one wire from the switch to the positive terminal of the battery or Psu and the other wire to the positive input of the 12v to 5v converter. putting a switch across a negative wire will never work switchs are designed to work on positive current only (worked this out by many years of tial and error work with circuits )